1. Field of the Invention
The present invention relates to circuitry for testing alarm switches used in overfill control systems of tank barges. The invention relates in particular to test systems usable in connection with dockside facility alarm and load/unload control systems.
2. Description of the Prior Art
Typically, the tanks of tank barges are equipped with overfill control systems which include liquid level sensing devices for detecting when significant levels of liquid lading in the tank are reached. Each detection device includes one or more normally-closed switches, which are opened when the liquid level reaches a corresponding level. Such devices typically include an overfill switch, which corresponds to an overfill condition of the tank, and may optionally include a high level switch, corresponding to a liquid level below the overfill condition but approaching it. These level sensing devices are used during the loading of the tank at a dockside facility.
There is typically no power source on the barge, for safety reasons in the event of highly volatile or combustible ladings. Accordingly, the level sensing device is coupled to a connector which can be connected to a dockside test and alarm system when the barge is at a dockside facility for loading or unloading. The dockside system applies a low current to each level sensing switch which, by government regulation, must normally be closed. The barge is typically provided with a manual system for operating the level sensing device to simulate an overfill or alarm condition and thereby open the switch. The tankerman performs this manual test while connected to the dockside facility and, if the switch opens properly the current will be interrupted. If the current is not interrupted, this indicates that there is a defect in the level sensing system and the barge will not be permitted to be loaded until the situation is remedied. This can be very time consuming and expensive, particularly where a barge has multiple tanks, since it may not be apparent which is the source of the problem, so each may have to be checked. During loading the level sensing device remains connected to the dockside alarm system and, if an alarm condition occurs during loading, the filling equipment will automatically be shut down.
While, by government regulation, there is a maximum voltage and current which can be applied to the barge fill level switches by the dockside alarm and test system so as to maintain an intrinsically safe condition on the barge, there is no minimum. Thus, different dock facilities utilize different voltages and currents, some of which currents can be as low as 0.1 mA (100 microamps). A signal current of 100 microamps may be insufficient to reliably detect tank barge fill level switch opening during certain conditions, such as high humidity conditions. If, for example, the maximum output voltage is 9.0 VDC, an impedance of as much as 90,000 ohms between the fill level switch conductors can result in a 100 microamp leakage current. The presence of this leakage current prevents the dock facility from detecting the barge level fill level switch opening.
Midland Manufacturing Company sells a transportable high level and overfill alarm system, which can be carried onto a barge and has its own built-in battery power supply for applying a test signal to the fill level switch circuits for purposes of testing same. It can also be connected to a dockside facility for actuating the dockside alarm and automatic shutdown system in the event of an overfill condition during loading. It applies a test signal current which is high enough to reliably detect level sensing switch opening in any conditions. It also uses optocouplers for optically isolating the tank alarm circuitry from the dockside facility alarm panel when the unit is connected to the dockside facility. More particularly, the light source of the optocoupler is connected in circuit with the barge level sensing switch, while the light-responsive switching element of the optocoupler is connected in circuit with the coil of a relay, the contacts of which are connected with the dockside facility, so that the condition of the relay contacts mirrors the condition of the barge level sensing switch.
While this prior transportable system works well, it has certain drawbacks. It incorporates its own alarm system, including strobe lights and a siren and it is rather heavy and bulky and may require two men to carry on board. Furthermore, it is rather expensive, costing several thousand dollars. Also, because it uses logic gate optocouplers, it requires two separate supply voltages, one to provide the test signal to the barge level sensing switches and another supply voltage to the logic circuitry. This requires that the system be connected to the power supply during the entire time that it is in operation and, since the optocouplers have a relatively high output leakage current of about 500 microamps and require a logic supply current of 4.8 mA, an ON-OFF switch is necessitated to avoid undue battery drain.